In this project period, we have developed means of stimulating single neurons in the cortex. We have deployed a dual-laser custom microscope capable of imaging and stimulation. To make cortical neurons accessible for imaging activity and stimulation, we have developed an approach to express of soma-targeted ChrimsonR (stChrimsonR) combined with expression of GCaMP7s. This allows high efficiency all-optical stimulation at 1030 nm (laser repetition rate: 500 kHz) and imaging at 920 nm (laser repetition rate: 80 MHz). To assess the effect of stimulating spatial and temporal patterns, we have done preliminary work to show which V1 spikes generated by a visual stimulus are used by the animal in detection behavior. We have measured the effect of inhibiting V1 at different times relative to stimulus onset. Resulaj et al. (2018 eLife) previously reported that only early spikes of V1 (in the first 40-80 ms of V1 neurons stimulus response) were used for visual discrimination, but they observed no effect on visual detection. We used a near-threshold change detection task, in VGAT-ChR2 mice that detected a contrast increment of a visual grating. One-photon optical stimulation was delivered to the surface of visual cortex. We found the same effect holds for visual detection: only the early spikes of V1 responses (within the first 80ms) are used.